Find the surface-radius of earth, Mathematics

a) The distance d that can be seen from horizon to horizon from an airplane varies directly as the square root of the altitude h of the airplane. If d = 213 km for h = 3950 m, find d for h = 5250 m.

b) The volume V of a given mass of gas varies directly as the temperature T and inversely as the pressure P. If V = 250 cm3 when T = 200° C and P = 130 kPa, what is the volume when T = 170°C and P = 130 kPa?

c) The acceleration of gravity g on a satellite in orbit around the earth varies inversely as the square of its distance r from the center of the earth.

If g = 8.7 m/s2 for a satellite at an altitude of 400 km above the surface of the earth, find g if it is 1000 km above the surface.